In the past two decades, fragmentation of concretions inside a living body by focused shockwaves from outside the body was established as a method of treatment. Recent research is also developing for the application of shockwave treatment in the field of orthopedics and in pathological tissue ablation as well as in other different types of treatment.
The mechanisms by which focused shockwaves disintegrate stones in Extracorporeal Shock Wave Lithotripsy are still not well understood. However, several mechanisms for stone fragmentation have been proposed and documented in the literature.
The shockwave pulse is comprised of a positive peak pressure up to about 120 MPa, which lasts for up to about 2 microseconds followed by negative peak pressure up to about 20 MPa with about 2 to 8 microseconds duration.
It is further known in the art that the negative pressure induces transient cavitation bubbles around the focal point. Ensuing pulses cause these cavitation bubbles to collapse. When bubbles collapse adjacent to a solid surface like a stone, it will take place asymmetrically leading to the formation of high speed, liquid micro jets that hit the stone surface and cause cracking and fragmentation. Only a percentage of these micro jets is directed to the stone while the remaining part is consumed by the adjacent tissues leading to tissue damage.
It is also mentioned in the literature that the conditions required for fracturing stones include one or more of the following: compression and release, tension or spall and cavitation induced stress. Fragmentation involves separation of crystal layers and fracture and cleavage of crystals. The disintegration of stones occurs by the progressive initiation of cracks and their stepwise extension through the material. Brittle materials fail under compressive shock loading by initiation and growth of micro cracks from internal defects such as pores or inclusion or from material boundaries such as interfaces with organic or fibrous material or grain boundaries.
With repeated pulses the micro cracks grow on a prospective spall plane and they coalesce on reaching a critical length creating a fragment. Under pressure, the micro cracks grow in the axial plane; i.e. the failure is in the direction of maximum applied compression that is the direction of shockwave propagation. On the other hand, under tension, micro cracks grow on a plane perpendicular to the direction of applied tension; i.e. perpendicular to the direction of the shockwave propagation.